This invention relates to apparatus and methods for shaping tubes and, in particular, to shaping glass tubes intended for use in the manufacture of optic fibers and for doing so semi-automatically or automatically.
One process for the manufacture of optical fibers is referred to as the modified chemical vapor deposition (MCVD) process in which the internal wall of a glass cylinder (also referred to herein as the xe2x80x9cstarter tubexe2x80x9d or xe2x80x9cpreform tubexe2x80x9d) is coated with uniform layers of reactants and gas vapors to form the optic fibers. To ensure the proper and uniform flow of gases within the starter tube, it is desirable to join, or fuse, the starter tube to an exhaust tube prior to initiating the actual fiber optic manufacturing process.
For optimum results the exhaust tube should provide smooth, continuous flow for the gases escaping from the starter tube to, and through, the exhaust tube. To accomplish this result, the exhaust tube must generally have a larger diameter than the starter tube. However, at their interface and mating ends the exhaust tube must mesh smoothly and continuously with the starter tube and must have a profile which aids in the smooth flow of gases out of the starter tube.
It is also noted that each exhaust tube is intended to be joined (or fused) to a starter tube and that the two xe2x80x9ccombinedxe2x80x9d tubes are then operated as a unit. Typically, the combined tubes are mounted in an apparatus in which they are made to rotate for many hours while gases and reactants are being injected into the starter tube under intense heat conditions, for forming uniform layers of optic fibers. To ensure the formation of even and uniform layers, it is important that the two tubes be aligned very accurately; (i.e., have a common center line) throughout the process.
In some presently known systems the end of an exhaust tube designed to mate with a starter tube is shaped manually using a graphite forming tool (e.g., a paddle), or like manual equipment. This process is an xe2x80x9cartxe2x80x9d dependent on the skills and techniques of the artisan shaping the mating end and interface of the exhaust tube. This is undesirable and problematic because tubes shaped manually have little uniformity and dimensional reproducibility. As a result, numerous defective exhaust tubes are produced. Equally problematic is that, even when an exhaust tube is not defective, the mating of an exhaust tube formed by the xe2x80x9cpaddlexe2x80x9d method with a starter tube is subject to alignment problems since no two exhaust tubes will normally be dimensionally equal.
In other presently known methods, a mold is applied to the exterior section of a tube, a plug is inserted into the end of the tube and air is injected within the tube to support the inner cavity of the tube during molding. Some materials, especially pure silica, require a great deal of air pressure to maintain the shape of the tube when the when the tube is heated to the point that it is malleable. In addition, increasingly greater levels of air pressure must be generated as the thickness of the glass wall of the tube increases. Because the glass cools very quickly when air is injected, the forming operation must be completed before the glass becomes too cool to form. As the required air pressure level increases, it becomes more difficult to complete the forming/molding operation before the glass cools. As a consequence, the heating/molding/air injection/ forming process must be repeated until the glass is finally formed/shaped as required. This subjects the process to errors, which is unacceptable. Elimination of the need to inject air is desirable as it eliminates the need for these multiple forming operations.
Applicants"" invention resides, in part, in the recognition that the prior art methods for shaping the exhaust tube are directed primarily at applying shaping forces to the exterior portions of the exhaust tube. Applicants recognized that it is equally, if not more, important to directly control the shaping of the inner surface of the exhaust tube.
Accordingly, the present invention is directed to apparatus and methods for more accurately shaping the inner surface of the end portion of an exhaust tube selected to mesh with a starter tube. This includes the placement of an internal mold, or a xe2x80x9ctoolxe2x80x9d, within the selected end section of the tube selected to be shaped. The use of an xe2x80x9cinternalxe2x80x9d mold reduces variations when using the present paddle, or pallet, method and it eliminates the need to inject air (and an end plug) when applying external molds to shape the tube.
The invention also includes a novel tool (xe2x80x9cmoldxe2x80x9d) for shaping the desired end section of a tube. In accordance with one aspect of the invention, the tool (xe2x80x9cmoldxe2x80x9d) has a variable configuration and may be selectively set to one configuration in which it is fully open or extended, or it may be set to another configuration in which it is closed or collapsed. When the tool is in its open, or extended, configuration, it functions as a mold and it then may be used to shape the inner surface of the tube. When the tool is in its closed, or collapsed, configuration, it is easily insertable into the tube or withdrawable from within the tube.
In accordance with one aspect of the invention, a collapsible tool is inserted into the end of an exhaust tube to be shaped. The collapsible tool can assume either a fully open, (i.e., extended) configuration or a closed (i.e., collapsed) configuration. To shape a selected end of an exhaust tube, the tool may be inserted into the tube either in its open or closed configuration. However prior to shaping the tube end, the tool is set to its open, extended configuration. Thereafter, heat is supplied to the tube end until the tube is rendered malleable. When the tube is rendered malleable, the exterior of the tube may be either manually compressed by means of a paddle or a mold may be applied to the exterior of the tube pushing the inner surface of the tube against the exterior surface(s) of the extended tool. The inner surface of the tube will then generally conform to the exterior shape of the extended tool. Thus, in apparatus according to the invention, since the internal mold defines the desired internal shape, many different external shaping means may be employed and the examples of external shaping means discussed below are merely illustrative.
In accordance with another aspect of the invention, a non-collapsible, oblate, cone-like mold may be inserted via one end of the tube and positioned, within the tube, at the tube end to be shaped. The tube end to be shaped can then be heated until the tube end is malleable and the tube can then be shaped manually, or with a mold. After shaping, the mold is removed via the one end of the tube.
The proposed method of controlling the inner surface of the tube during shaping is more robust than the current paddle method and results in a more accurately dimensioned and consistently dimensioned exhaust tube. The proposed method is less labor intensive and maintenance intensive than the prior art method and will result in yield improvements and cost reductions.